- Email: [email protected]
Thermal stability improvement and optical band gap behavior in Ge2Te films by Mg-doping
Accepted Manuscript Thermal stability improvement and optical band gap behavior in Ge2Te films by Mgdoping Yang Luo, Guoxiang Wang, Peng Liu, Zhenglai Wang, Jiaxing Wang, Ting Gu PII:
S0042-207X(17)30217-8
DOI:
10.1016/j.vacuum.2017.04.008
Reference:
VAC 7368
To appear in:
Vacuum
Received Date: 17 February 2017 Accepted Date: 4 April 2017
Please cite this article as: Luo Y, Wang G, Liu P, Wang Z, Wang J, Gu T, Thermal stability improvement and optical band gap behavior in Ge2Te films by Mg-doping, Vacuum (2017), doi: 10.1016/ j.vacuum.2017.04.008. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Thermal stability improvement and optical band gap behavior in Ge2Te films by Mg-doping Yang Luo1,2, Guoxiang Wang1,2*, Peng Liu1,2, Zhenglai Wang1,2, Jiaxing Wang1,2,
1
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Ting Gu1,2 Laboratory of Infrared Material and Devices, The Advanced Technology Research Institute, Ningbo University, Ningbo 315211, China
Key Laboratory of Photoelectric Detection Materials and Devices of Zhejiang
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2
Province，Ningbo, 315211, China
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Abstract
The thermal stability and its crystallization behavior in Ge-Mg-Te films were investigated. The results reveal that the bond combination among Mg, Ge and Te has occurred in Mg-Ge-Te. The formed Mg-Ge and Mg-Te phases increase the
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crystallization temperature, crystalline resistance and optical band gap. The ratios between amorphous and crystalline states were increased to 108. The crystal phase was changed from Ge2Te to Te. The proper Mg serves as a center for suppression of
application.
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the Te crystal growth without phase separation for high-stable phase change memory
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Keywords: Thin films; Crystal structure; Thermal properties; Electrical properties
*
Electronic mail: [email protected] (G.Wang) 1
ACCEPTED MANUSCRIPT 1、 、Introduction Phase change memory (PCM) is based on the change in reflectivity or resistivity of phase-change materials, exploiting the reversible transformation between the amorphous and crystalline states under laser or electrical pulses [1]. In addition, the
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PCM has been a great concern owing to being compatible with existing complementary metal-oxide semiconductor (CMOS) technologies, a remarkable speed of read/write (20 ns / 10 ns), a longer cycle life (> 1012), et al. [2]. However, the
SC
development of PRAM is limited to its large power consumption and poor thermal stability. Presently, the main studies focus on how to optimize the material by
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increasing crystalline resistance and crystallization temperature to improve the physical properties [3]. It is well known that conventional Ge2Sb2Te5 (GST) film is a typical pseudobinary material that inherits the good thermal stability of GeTe and the fast phase-change ability of Sb2Te3, showing overall qualified performances. An
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effective way to improve the physical properties is to add specific foreign elements. For example, J.H. Park et al. [4] reported that the grain size of GST was decreased from 10.3 to 1nm and the crystalline resistance was increased by two orders of
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magnitude. F. Rao et al. [5] reported that the grain size of Si-Sb2Te3 films was
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decreased from 40 to 10 nm and the crystalline resistance was increased by 2 times. C. Peng et al. [6] revealed that the crystallization temperature of GeTe was markedly improved by introducing W bonded to Ge and Te atoms. Our previous studies on the crystallization of Mg-doped GST [7] and Sb-Te [8]
films found that Mg, Sb, and Te atoms formed amorphous Mg-Sb and Mg-Te phases, increasing the crystallization temperature and crystalline resistance. Still, there was no comprehensive study of the influence of Mg on the structure and properties of the Ge-Te films. In this paper, novel Mg-doped Ge2Te films were prepared and the 2
ACCEPTED MANUSCRIPT crystallization behavior was investigated. With the help of in situ sheet resistance measurement, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission spectra, the crystallization effects on Mg-Ge2Te films were considered carefully.
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2、 、Experimental
Mg-doped Ge2Te films of 200 nm thickness were deposited on quartz and SiO2/Si (100) substrates by a magnetron co-sputtering method using individual Mg
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and Ge2Te targets. The chamber was evacuated to 5×10−5 Pa before Ar gas was introduced to a pressure of 0.35 Pa for the film deposition. The radio frequency power
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(Prf) was fixed at 60 W and applied to a Ge2Te target. The amount of Mg in the Ge2Te films was adjusted by varying the direct current power (Pdc) from 0, 3, 8, 11 and 16 W applied to the Mg target. The stoichiometry of the as-deposited films and chemical bonding states of the elements were confirmed by XPS. The sheet resistance of the
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as-deposited films was measured in situ using a four point probe in a vacuum chamber built in-house as a function of temperature (non-isothermal). The structure of as-deposited and annealed films was examined by XRD. The optical transmittance
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(Top) of the thin films in the spectral range 800–2500nm was obtained using
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Perkin-Elmer Lambda 950 UV-VIS-NIR spectrophotometer. 3、 、Results and discussion Fig.1 shows XRD patterns of undoped and Mg-doped Ge2Te films annealed at
various temperatures for 3min. No diffraction peaks are observed for Ge2Te films annealed at 150 and 200 ºC as shown in Fig.1(a), indicating that Ge2Te films are kept a good amorphous nature similar as the as-deposited amorphous state. However, with the increase of annealing temperature from 250 to 350 ºC, the film begins to crystallize gradually. Crystallization peaks are well-fitted using a rhombohedral 3
ACCEPTED MANUSCRIPT GeTe-phase (JCPDS no.47-1079) appeared at 400 ºC. As for Mg-doped Ge2Te films, the Mg8.7(Ge2Te)91.3 film exhibits an amorphous state before and after annealing at 200 ºC as shown in Fig.1(b). Then, peak-(101) first appears in the Mg8.7(Ge2Te)91.3 film annealed at 250 ºC. As the annealing temperature increases to 400 ºC, the intense
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characteristic peaks-(100), (101), (102), (110) and (201) indicate that the precipitated crystalline phase is Te, revealing that Mg-doping changes the crystal type from GeTe into Te. With Mg-doping concentration increasing further, the amorphous phase still
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remains in Mg14.2(Ge2Te)85.8, Mg20.6 (Ge2Te)79.4 and Mg28.9(Ge2Te)71.1 films even that the annealing temperature is increased to 350 ºC. Differently, the Te crystalline peaks
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can be observed in the Mg14.2(Ge2Te)85.8 film annealed at 400 ºC as shown in Fig. 1(c). While for Mg20.6(Ge2Te)79.4 and Mg28.9(Ge2Te)71.1 films as shown in Figs.1(d)-(e), the crystallization peaks become weak gradually with increasing Mg content, more importantly, peaks-(102) and (110) disappear in Fig.1(d), while peaks-(100) and (201)
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disappear in Fig.1(e), implying that the crystallization process of Mg-Ge2Te film is suppressed by Mg-doping, resulting in the reduced grain size from 10 to 2 nm calculated by Scherrer equation and an enhancement of thermal stability. This can be
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further confirmed in R-T test.
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In order to confirm the role of Mg in Ge2Te thin film, the curves of the resistance versus the annealing temperature (R–T) with a heating rate of 40 K/min was shown in Fig.2. Sheet resistances are decreased gradually with increasing temperature up to their respective crystallization temperature (Tc), where an abrupt drop of the sheet resistance confirms the phase transition from amorphous to crystalline phases. Moreover, the Tc of Mg8.7(Ge2Te)91.3, Mg14.2(Ge2Te)85.8, Mg20.6 (Ge2Te)79.4 and Mg28.9(Ge2Te)71.1 films are estimated to be at ~220, ~235, ~252 and ∼255 ºC, respectively, which are higher than that of pure Ge2Te (~205 ºC). A higher 4
ACCEPTED MANUSCRIPT crystallization temperature is desired to prolong the archival life. On the other hand, the resistance of Mg-Ge2Te films increases with increasing Mg concentration. The higher crystalline resistance of Mg-Ge2Te films could contribute to lower power consumption for PCM device. The amorphous/crystalline
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resistance ratio (Ra/Rc) remains at about 107-108 as shown in Fig.2(b), which is helpful to obtain the large ON/OFF ratio.
To analyze the chemical bonding features, XPS Te 3d and Ge 3d spectra for the
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Ge2Te, Mg8.7(Ge2Te)91.3 and Mg14.2(Ge2Te)85.8 films were investigated as shown in Figs. 3(a)–(b). In the case of Te 3d and Ge 3d spectra, the both peak positions shift to
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lower binding energy after Mg-doping. The negative shift in the binding energy increases with decreasing electronegativity of the neighbouring atom since the electro-negativity of Mg (1.31) is smaller than that of Ge (2.01) and Te (2.1). Consequently, the decrease of the binding energy of Te 3d and Ge 3d is due to the fact
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that part of the Te and Ge atoms in Ge-Te bonds are replaced by Mg atoms, forming Mg-Te and Mg-Ge bonds. The formation of covalent bonds in Mg-doped Ge2Te films
stability.
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could lead to an increase in Tc, thus an enhancement in the amorphous thermal
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As for the phase change material, a large optical band-gap is demanded in order to reduce the threshold current. We measured UV-VIS-NIR spectra of the amorphous Ge2Te and Mg14.2(Ge2Te)85.8 films in order to determine optical band gap (Eopt), and the plots of (αhυ)1/2-(hυ) were shown in Fig.4. By extrapolating the linear portion of the curves to zero absorption, the indirect Eopt valus are determined to increase from 0.62 for Ge2Te to 0.84 eV for Mg14.2(Ge2Te)85.8, which is slightly larger than that of GST (~0.697 eV) reported in Ref. [9], indicating that the incorporation of Mg could reduce the number of unsaturated bonds in amorphous films just as the formation of 5
ACCEPTED MANUSCRIPT Mg-Ge and Mg-Te, and thus decrease the density of localized states in the band structure and consequently increase the Eopt [10]. 4、 、Conclusions Novel Mg-Ge-Te films were prepared and the phase-change behavior was
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investigated. The bond combination among Mg, Ge and Te has occurred. The formed Mg-Ge and Mg-Te bonds present in the amorphous state. The amorphous Mg-Ge and Mg-Te phases increase the crystallization temperature, crystalline resistance and
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optical band gap. The Mg-Ge-Te films exhibit a stable Te crystalline phase without phase separation and the grain growth is suppressed with the formed Mg-Ge and
phase change memory application. Acknowledgements
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Mg-Te phases. The excellent properties make Mg-Ge2Te films be a candidate for
This work was financially supported by the National Natural Science Foundation
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of China (Grant No. 61604083), the Natural Science Foundation of Zhejiang Province, China (Grant No. LQ15F040002), and was sponsored by the K. C. Wong Magna Fund
References
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at Ningbo University.
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[1] M. Wutting, N. Yamada, Phase-change materials for rewriteable data storage, Nat. Mater. 6 (2007) 824-832. [2] C.E. Giusca, V. Stolojan, J. Sloan, B. Buchner, S.R.P. Silva. Confined crystals of the smallest phase-change material, Nano Lett. 13 (2013) 4020-4027. [3] D. Loke, T.H. Lee, W.J. Wang, T.C. Chong, S.R. Elliott. Breaking the speed limits of phase-change memory, Science 336(2012) 1566-1569. [4] J. H. Park, S.W. Kim, J. H. Kim, Z. Wu, S. L. Cho, D. Ahn, D. H. Ahn, J. M. Lee, S. U. Nam, D.H. Ko，Reduction of RESET current in phase change memory devices 6
ACCEPTED MANUSCRIPT by carbon doping in GeSbTe films, J. Appl. Phys. 117 (2015) 115703. [5] F. Rao, Z.T. Song, K. Ren, X.L. Zhou, Y. Cheng, L.C. Wu, B. Liu, Si-Sb-Te materials for phase change memory applications, Nanotechnology 22 (2011) 145702. [6] C. Peng, F. Rao, L.C. Wu, Z.T. Song, Y.F. Gu, D. Zhou, H.J. Song, P.X. Yang, J.H.
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Chu, Homogeneous phase W-Ge-Te material with improved overall phase-change properties for future nonvolatile memory, Acta Mater. 74(2014) 49-57.
[7] J. Fu, X. Shen, Q.H. Nie, G.X. Wang, L.C. Wu, S.X. Dai, T.F. Xu, R.P. Wang,
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Crystallization characteristics of Mg-doped Ge2Sb2Te5 films for phase change memory application, Appl. Surf. Sci. 24(2013) 269-272.
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[8] J.J. Li, G.X Wang, J. Li, X. Shen, Y.M. Chen, R.P. Wang, T.F. Xu, Q.H. Nie, S.X. Dai, Y.G. Lu, X.S. Wang, Fast crystallization and low-power amorphization of Mg-Sb-Te reversible phase-change films, CrystEngComm, 16(2014) 7401-7405. [9] G.X. Wang, X. Shen, Q.H. Nie, R.P. Wang, L.C. Wu, Y.G. Lv, F. Chen, J. Fu,
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S.X. Dai, J. Li, Improved thermal and electrical properties of Al-doped Ge2Sb2Te5 films for phase-change random access memory, J. Phys. D. Appl. Phys. 45 (2012) 375302-375307.
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[10] W. Welnic, A. Pamungka, R. Delemple, C. Steimer, S. Blugel, M. Wutting,
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Unravelling the interplay of local structure and physical properties in phase-change materials, Nat. Mater. 5(2006) 56-62.
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ACCEPTED MANUSCRIPT Figure captions Fig. 1 XRD patterns of (a) Ge2Te, (b) Mg8.7(Ge2Te)91.3, (c) Mg14.2(Ge2Te)85.8, (d) Mg20.6(Ge2Te)79.4 and (e) Mg28.9(Ge2Te)71.1 films before and after isothermal annealing
(b) Ra/Rc resistance ratio of Mg-Ge2Te films
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Fig. 2 (a) Sheet resistance of Mg doped Ge2Te films as a function of temperature and
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Fig.3 XPS spectra for as-deposited Ge2Te, Mg8.7(Ge2Te)91.3 and Mg14.2(Ge2Te)85.8 films: (a) Te 3d and (b) Ge 3d
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Fig.4 The plots of (αhυ)1/2-(hυ) for amorphous Mg-Ge2Te films
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250°C 200°C 150°C as dep.
40
50
0
250 C 2000C
as dep.
60
10
20
30
50
350°C
(100)
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300°C
250°C
200°C
20
30
40
50
Diffraction (2θ)
20
300°C 250°C 200°C as dep.
30
Mg28.9(Ge2Te)71.1
(100)
Te
Intensity (a.u.)
400°C 350°C 300°C
250°C
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10
40
Diffraction (2θ)
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(e)
10
60
200°C
as dep. 20
30
40
Diffraction (2θ)
Fig. 1
50
400°C 350°C
as dep. 10
60
Mg20.6(Ge2Te)79.4
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Intensity (a.u.)
400°C
Te
(201)
(d)
Mg14.2(Ge2Te)85.8 (201)
(102) (110)
(101)
40
Diffraction (2θ)
Intensity (a.u.)
(100)
Te
(201)
3000C
Diffraction (2θ) (c)
(102) (110)
0
350 C
(101)
30
4000C
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300°C
20
Mg8.7(Ge2Te)91.3
Intensity (a.u.)
(042)
350°C
10
Te
(101)
(b)
400°C
(100)
Ge2Te (220)
Intensity (a.u.)
(021)
GeTe
(202)
(a)
60
50
60
ACCEPTED MANUSCRIPT (a)
160
10
10
Tc
8
6
10
Mg8.7(Ge2Te)91.3 Mg14.2(Ge2Te)85.8
40
Mg20.6(Ge2Te)79.4
2
10
80
Mg28.9(Ge2Te)71.1
0
-50
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100 150 200 250 300 350
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Ge2Te
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570
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Binding energy (eV)
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Mg14.2(Ge2Te)85.8
0.5
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(αhv) (eV.cm )
Ge2Te
300
200
100
0 0.4
0.62eV 0.6
30
32
Binding energy (eV)
Fig.3
400
25
30
Intensity (a.u.)
Intensity (a.u.) 568
20
Ge 3d
Mg8.7(Ge2Te)91.3 Mg14.2(Ge2Te)85.8
15
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Te 3d
10
Mg content ( at. %)
Fig. 2
(a)
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4
10
10
Rc
Ge2Te
6
10
120
Ra/Rc (10 )
Sheet resistance (Ω/ )
(b)
Ra
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10
0.8
0.84eV 1.0
hv(eV) Fig. 4
1.2
34
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The bond combination among Mg, Ge and Te has occurred in Mg-Ge-Te. The formed Mg-Ge and Mg-Te phases increase the Tc, and Rc. The crystal phase was changed from Ge2Te to Te. The proper Mg serves as a center for suppression of the Te crystal growth. The optical gap was increased due to the formed Mg-Ge and Mg-Te phases.
S0042-207X(17)30217-8
DOI:
10.1016/j.vacuum.2017.04.008
Reference:
VAC 7368
To appear in:
Vacuum
Received Date: 17 February 2017 Accepted Date: 4 April 2017
Please cite this article as: Luo Y, Wang G, Liu P, Wang Z, Wang J, Gu T, Thermal stability improvement and optical band gap behavior in Ge2Te films by Mg-doping, Vacuum (2017), doi: 10.1016/ j.vacuum.2017.04.008. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Thermal stability improvement and optical band gap behavior in Ge2Te films by Mg-doping Yang Luo1,2, Guoxiang Wang1,2*, Peng Liu1,2, Zhenglai Wang1,2, Jiaxing Wang1,2,
1
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Ting Gu1,2 Laboratory of Infrared Material and Devices, The Advanced Technology Research Institute, Ningbo University, Ningbo 315211, China
Key Laboratory of Photoelectric Detection Materials and Devices of Zhejiang
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2
Province，Ningbo, 315211, China
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Abstract
The thermal stability and its crystallization behavior in Ge-Mg-Te films were investigated. The results reveal that the bond combination among Mg, Ge and Te has occurred in Mg-Ge-Te. The formed Mg-Ge and Mg-Te phases increase the
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crystallization temperature, crystalline resistance and optical band gap. The ratios between amorphous and crystalline states were increased to 108. The crystal phase was changed from Ge2Te to Te. The proper Mg serves as a center for suppression of
application.
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the Te crystal growth without phase separation for high-stable phase change memory
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Keywords: Thin films; Crystal structure; Thermal properties; Electrical properties
*
Electronic mail: [email protected] (G.Wang) 1
ACCEPTED MANUSCRIPT 1、 、Introduction Phase change memory (PCM) is based on the change in reflectivity or resistivity of phase-change materials, exploiting the reversible transformation between the amorphous and crystalline states under laser or electrical pulses [1]. In addition, the
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PCM has been a great concern owing to being compatible with existing complementary metal-oxide semiconductor (CMOS) technologies, a remarkable speed of read/write (20 ns / 10 ns), a longer cycle life (> 1012), et al. [2]. However, the
SC
development of PRAM is limited to its large power consumption and poor thermal stability. Presently, the main studies focus on how to optimize the material by
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increasing crystalline resistance and crystallization temperature to improve the physical properties [3]. It is well known that conventional Ge2Sb2Te5 (GST) film is a typical pseudobinary material that inherits the good thermal stability of GeTe and the fast phase-change ability of Sb2Te3, showing overall qualified performances. An
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effective way to improve the physical properties is to add specific foreign elements. For example, J.H. Park et al. [4] reported that the grain size of GST was decreased from 10.3 to 1nm and the crystalline resistance was increased by two orders of
EP
magnitude. F. Rao et al. [5] reported that the grain size of Si-Sb2Te3 films was
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decreased from 40 to 10 nm and the crystalline resistance was increased by 2 times. C. Peng et al. [6] revealed that the crystallization temperature of GeTe was markedly improved by introducing W bonded to Ge and Te atoms. Our previous studies on the crystallization of Mg-doped GST [7] and Sb-Te [8]
films found that Mg, Sb, and Te atoms formed amorphous Mg-Sb and Mg-Te phases, increasing the crystallization temperature and crystalline resistance. Still, there was no comprehensive study of the influence of Mg on the structure and properties of the Ge-Te films. In this paper, novel Mg-doped Ge2Te films were prepared and the 2
ACCEPTED MANUSCRIPT crystallization behavior was investigated. With the help of in situ sheet resistance measurement, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and transmission spectra, the crystallization effects on Mg-Ge2Te films were considered carefully.
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2、 、Experimental
Mg-doped Ge2Te films of 200 nm thickness were deposited on quartz and SiO2/Si (100) substrates by a magnetron co-sputtering method using individual Mg
SC
and Ge2Te targets. The chamber was evacuated to 5×10−5 Pa before Ar gas was introduced to a pressure of 0.35 Pa for the film deposition. The radio frequency power
M AN U
(Prf) was fixed at 60 W and applied to a Ge2Te target. The amount of Mg in the Ge2Te films was adjusted by varying the direct current power (Pdc) from 0, 3, 8, 11 and 16 W applied to the Mg target. The stoichiometry of the as-deposited films and chemical bonding states of the elements were confirmed by XPS. The sheet resistance of the
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as-deposited films was measured in situ using a four point probe in a vacuum chamber built in-house as a function of temperature (non-isothermal). The structure of as-deposited and annealed films was examined by XRD. The optical transmittance
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(Top) of the thin films in the spectral range 800–2500nm was obtained using
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Perkin-Elmer Lambda 950 UV-VIS-NIR spectrophotometer. 3、 、Results and discussion Fig.1 shows XRD patterns of undoped and Mg-doped Ge2Te films annealed at
various temperatures for 3min. No diffraction peaks are observed for Ge2Te films annealed at 150 and 200 ºC as shown in Fig.1(a), indicating that Ge2Te films are kept a good amorphous nature similar as the as-deposited amorphous state. However, with the increase of annealing temperature from 250 to 350 ºC, the film begins to crystallize gradually. Crystallization peaks are well-fitted using a rhombohedral 3
ACCEPTED MANUSCRIPT GeTe-phase (JCPDS no.47-1079) appeared at 400 ºC. As for Mg-doped Ge2Te films, the Mg8.7(Ge2Te)91.3 film exhibits an amorphous state before and after annealing at 200 ºC as shown in Fig.1(b). Then, peak-(101) first appears in the Mg8.7(Ge2Te)91.3 film annealed at 250 ºC. As the annealing temperature increases to 400 ºC, the intense
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characteristic peaks-(100), (101), (102), (110) and (201) indicate that the precipitated crystalline phase is Te, revealing that Mg-doping changes the crystal type from GeTe into Te. With Mg-doping concentration increasing further, the amorphous phase still
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remains in Mg14.2(Ge2Te)85.8, Mg20.6 (Ge2Te)79.4 and Mg28.9(Ge2Te)71.1 films even that the annealing temperature is increased to 350 ºC. Differently, the Te crystalline peaks
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can be observed in the Mg14.2(Ge2Te)85.8 film annealed at 400 ºC as shown in Fig. 1(c). While for Mg20.6(Ge2Te)79.4 and Mg28.9(Ge2Te)71.1 films as shown in Figs.1(d)-(e), the crystallization peaks become weak gradually with increasing Mg content, more importantly, peaks-(102) and (110) disappear in Fig.1(d), while peaks-(100) and (201)
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disappear in Fig.1(e), implying that the crystallization process of Mg-Ge2Te film is suppressed by Mg-doping, resulting in the reduced grain size from 10 to 2 nm calculated by Scherrer equation and an enhancement of thermal stability. This can be
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further confirmed in R-T test.
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In order to confirm the role of Mg in Ge2Te thin film, the curves of the resistance versus the annealing temperature (R–T) with a heating rate of 40 K/min was shown in Fig.2. Sheet resistances are decreased gradually with increasing temperature up to their respective crystallization temperature (Tc), where an abrupt drop of the sheet resistance confirms the phase transition from amorphous to crystalline phases. Moreover, the Tc of Mg8.7(Ge2Te)91.3, Mg14.2(Ge2Te)85.8, Mg20.6 (Ge2Te)79.4 and Mg28.9(Ge2Te)71.1 films are estimated to be at ~220, ~235, ~252 and ∼255 ºC, respectively, which are higher than that of pure Ge2Te (~205 ºC). A higher 4
ACCEPTED MANUSCRIPT crystallization temperature is desired to prolong the archival life. On the other hand, the resistance of Mg-Ge2Te films increases with increasing Mg concentration. The higher crystalline resistance of Mg-Ge2Te films could contribute to lower power consumption for PCM device. The amorphous/crystalline
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resistance ratio (Ra/Rc) remains at about 107-108 as shown in Fig.2(b), which is helpful to obtain the large ON/OFF ratio.
To analyze the chemical bonding features, XPS Te 3d and Ge 3d spectra for the
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Ge2Te, Mg8.7(Ge2Te)91.3 and Mg14.2(Ge2Te)85.8 films were investigated as shown in Figs. 3(a)–(b). In the case of Te 3d and Ge 3d spectra, the both peak positions shift to
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lower binding energy after Mg-doping. The negative shift in the binding energy increases with decreasing electronegativity of the neighbouring atom since the electro-negativity of Mg (1.31) is smaller than that of Ge (2.01) and Te (2.1). Consequently, the decrease of the binding energy of Te 3d and Ge 3d is due to the fact
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that part of the Te and Ge atoms in Ge-Te bonds are replaced by Mg atoms, forming Mg-Te and Mg-Ge bonds. The formation of covalent bonds in Mg-doped Ge2Te films
stability.
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could lead to an increase in Tc, thus an enhancement in the amorphous thermal
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As for the phase change material, a large optical band-gap is demanded in order to reduce the threshold current. We measured UV-VIS-NIR spectra of the amorphous Ge2Te and Mg14.2(Ge2Te)85.8 films in order to determine optical band gap (Eopt), and the plots of (αhυ)1/2-(hυ) were shown in Fig.4. By extrapolating the linear portion of the curves to zero absorption, the indirect Eopt valus are determined to increase from 0.62 for Ge2Te to 0.84 eV for Mg14.2(Ge2Te)85.8, which is slightly larger than that of GST (~0.697 eV) reported in Ref. [9], indicating that the incorporation of Mg could reduce the number of unsaturated bonds in amorphous films just as the formation of 5
ACCEPTED MANUSCRIPT Mg-Ge and Mg-Te, and thus decrease the density of localized states in the band structure and consequently increase the Eopt [10]. 4、 、Conclusions Novel Mg-Ge-Te films were prepared and the phase-change behavior was
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investigated. The bond combination among Mg, Ge and Te has occurred. The formed Mg-Ge and Mg-Te bonds present in the amorphous state. The amorphous Mg-Ge and Mg-Te phases increase the crystallization temperature, crystalline resistance and
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optical band gap. The Mg-Ge-Te films exhibit a stable Te crystalline phase without phase separation and the grain growth is suppressed with the formed Mg-Ge and
phase change memory application. Acknowledgements
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Mg-Te phases. The excellent properties make Mg-Ge2Te films be a candidate for
This work was financially supported by the National Natural Science Foundation
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of China (Grant No. 61604083), the Natural Science Foundation of Zhejiang Province, China (Grant No. LQ15F040002), and was sponsored by the K. C. Wong Magna Fund
References
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at Ningbo University.
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[1] M. Wutting, N. Yamada, Phase-change materials for rewriteable data storage, Nat. Mater. 6 (2007) 824-832. [2] C.E. Giusca, V. Stolojan, J. Sloan, B. Buchner, S.R.P. Silva. Confined crystals of the smallest phase-change material, Nano Lett. 13 (2013) 4020-4027. [3] D. Loke, T.H. Lee, W.J. Wang, T.C. Chong, S.R. Elliott. Breaking the speed limits of phase-change memory, Science 336(2012) 1566-1569. [4] J. H. Park, S.W. Kim, J. H. Kim, Z. Wu, S. L. Cho, D. Ahn, D. H. Ahn, J. M. Lee, S. U. Nam, D.H. Ko，Reduction of RESET current in phase change memory devices 6
ACCEPTED MANUSCRIPT by carbon doping in GeSbTe films, J. Appl. Phys. 117 (2015) 115703. [5] F. Rao, Z.T. Song, K. Ren, X.L. Zhou, Y. Cheng, L.C. Wu, B. Liu, Si-Sb-Te materials for phase change memory applications, Nanotechnology 22 (2011) 145702. [6] C. Peng, F. Rao, L.C. Wu, Z.T. Song, Y.F. Gu, D. Zhou, H.J. Song, P.X. Yang, J.H.
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Chu, Homogeneous phase W-Ge-Te material with improved overall phase-change properties for future nonvolatile memory, Acta Mater. 74(2014) 49-57.
[7] J. Fu, X. Shen, Q.H. Nie, G.X. Wang, L.C. Wu, S.X. Dai, T.F. Xu, R.P. Wang,
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Crystallization characteristics of Mg-doped Ge2Sb2Te5 films for phase change memory application, Appl. Surf. Sci. 24(2013) 269-272.
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[8] J.J. Li, G.X Wang, J. Li, X. Shen, Y.M. Chen, R.P. Wang, T.F. Xu, Q.H. Nie, S.X. Dai, Y.G. Lu, X.S. Wang, Fast crystallization and low-power amorphization of Mg-Sb-Te reversible phase-change films, CrystEngComm, 16(2014) 7401-7405. [9] G.X. Wang, X. Shen, Q.H. Nie, R.P. Wang, L.C. Wu, Y.G. Lv, F. Chen, J. Fu,
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S.X. Dai, J. Li, Improved thermal and electrical properties of Al-doped Ge2Sb2Te5 films for phase-change random access memory, J. Phys. D. Appl. Phys. 45 (2012) 375302-375307.
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[10] W. Welnic, A. Pamungka, R. Delemple, C. Steimer, S. Blugel, M. Wutting,
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Unravelling the interplay of local structure and physical properties in phase-change materials, Nat. Mater. 5(2006) 56-62.
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ACCEPTED MANUSCRIPT Figure captions Fig. 1 XRD patterns of (a) Ge2Te, (b) Mg8.7(Ge2Te)91.3, (c) Mg14.2(Ge2Te)85.8, (d) Mg20.6(Ge2Te)79.4 and (e) Mg28.9(Ge2Te)71.1 films before and after isothermal annealing
(b) Ra/Rc resistance ratio of Mg-Ge2Te films
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Fig. 2 (a) Sheet resistance of Mg doped Ge2Te films as a function of temperature and
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Fig.3 XPS spectra for as-deposited Ge2Te, Mg8.7(Ge2Te)91.3 and Mg14.2(Ge2Te)85.8 films: (a) Te 3d and (b) Ge 3d
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Fig.4 The plots of (αhυ)1/2-(hυ) for amorphous Mg-Ge2Te films
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250°C 200°C 150°C as dep.
40
50
0
250 C 2000C
as dep.
60
10
20
30
50
350°C
(100)
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300°C
250°C
200°C
20
30
40
50
Diffraction (2θ)
20
300°C 250°C 200°C as dep.
30
Mg28.9(Ge2Te)71.1
(100)
Te
Intensity (a.u.)
400°C 350°C 300°C
250°C
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10
40
Diffraction (2θ)
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(e)
10
60
200°C
as dep. 20
30
40
Diffraction (2θ)
Fig. 1
50
400°C 350°C
as dep. 10
60
Mg20.6(Ge2Te)79.4
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Intensity (a.u.)
400°C
Te
(201)
(d)
Mg14.2(Ge2Te)85.8 (201)
(102) (110)
(101)
40
Diffraction (2θ)
Intensity (a.u.)
(100)
Te
(201)
3000C
Diffraction (2θ) (c)
(102) (110)
0
350 C
(101)
30
4000C
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300°C
20
Mg8.7(Ge2Te)91.3
Intensity (a.u.)
(042)
350°C
10
Te
(101)
(b)
400°C
(100)
Ge2Te (220)
Intensity (a.u.)
(021)
GeTe
(202)
(a)
60
50
60
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160
10
10
Tc
8
6
10
Mg8.7(Ge2Te)91.3 Mg14.2(Ge2Te)85.8
40
Mg20.6(Ge2Te)79.4
2
10
80
Mg28.9(Ge2Te)71.1
0
-50
0
50
0
100 150 200 250 300 350
0
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Temperature ( C)
5
(b)
Ge2Te
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572
574
576
26
28
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Binding energy (eV)
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Mg14.2(Ge2Te)85.8
0.5
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(αhv) (eV.cm )
Ge2Te
300
200
100
0 0.4
0.62eV 0.6
30
32
Binding energy (eV)
Fig.3
400
25
30
Intensity (a.u.)
Intensity (a.u.) 568
20
Ge 3d
Mg8.7(Ge2Te)91.3 Mg14.2(Ge2Te)85.8
15
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Te 3d
10
Mg content ( at. %)
Fig. 2
(a)
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4
10
10
Rc
Ge2Te
6
10
120
Ra/Rc (10 )
Sheet resistance (Ω/ )
(b)
Ra
12
10
0.8
0.84eV 1.0
hv(eV) Fig. 4
1.2
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The bond combination among Mg, Ge and Te has occurred in Mg-Ge-Te. The formed Mg-Ge and Mg-Te phases increase the Tc, and Rc. The crystal phase was changed from Ge2Te to Te. The proper Mg serves as a center for suppression of the Te crystal growth. The optical gap was increased due to the formed Mg-Ge and Mg-Te phases.